1. Field of the Invention
The present invention mainly relates to a photosensitive-member driving mechanism employed in an image forming apparatus.
2. Description of the Related Art
As countermeasures against rotation variations (peripheral speed variations) of the photosensitive drums in conventional image forming apparatuses, there have been known first type systems for matching the phases of the rotation variations of plural photosensitive members for colored-image formation for reducing color shifts (refer to Japanese Unexamined Patent Publication No. Hei7(1995)-261499, for example) and second type systems for reducing the speed variations of photosensitive members through speed controlling of the motors for driving the photosensitive members (refer to Japanese Unexamined Patent Publication No. Hei10(1998)-78734, for example).
With the aforementioned first type systems, it is impossible to eliminate the distortions of images themselves and, even in cases of monochrome images, images formed on the photosensitive members are distorted in the direction of secondary scanning, which imposes a limit on the improvement of the image quality.
Further, in cases of image forming apparatuses including plural photosensitive members with different diameters, among color image forming apparatuses, color-shift adjustments can not be performed at all. Accordingly, in order to extend the life of the photosensitive member for black-color images and in order to increase the printing speed, if only the diameter of this photosensitive member is increased, this will make it impossible to eliminate color shifts.
With the aforementioned second type systems, there is a need for providing driving motors for controlling the speeds of the respective photosensitive drums and, further, there is a need for controlling the speeds thereof with higher accuracy, which has induced the problem of increase of the number of components of the apparatus and the necessity of complicated controlling.